The present patent application is a non-provisional application of International Application No. PCT/KR03/00779, filed Apr. 17, 2003.
(a) Field of the Invention
The present invention relates to an equalizer and a control method of the equalizer which are used for improving performance in a receiver under a multi-path communication environment of indoor, portable, and mobile reception.
(b) Description of the Related Art
In the ATSC (Advanced Television Systems Committee) standard which has been adopted as a digital television broadcasting standard in North America, there is a problem in that transmission signals are distorted under an indoor and mobile communication environment due to variable channel and multi-path effects. This reduces reception performance in a receiver such as a digital television.
In a digital communication system, data transmission is performed in symbol units through a restricted frequency band width, and pulse energy of a symbol may be dispersed into neighboring symbol pulses due to a time dispersion effect. This induces interference in the neighboring symbols. In addition, transmitted data is affected by various kinds of channel distortions, caused by such effects as a multi-path effect, a frequency offset effect, a phase jitter effect, and so on. These effects induce ISI (Inter-Symbol Interference) which may affect neighboring symbols of a transmission symbol in a digital communication system. Therefore, the ISI becomes a barrier to obtaining desired data in a receiver. In addition, most communication channels require an adaptive equalizer which may update tap coefficients adaptively with respect to time variation, since the above described distortions are variable.
FIG. 1 illustrates a block diagram of a conventional VSB (vestigial sideband) signal receiving system. In FIG. 1, a tuner 100 receives an RF signal from an antenna and synchronizes the RF signal with a local oscillation signal to transform the RF signal into an IF signal; an NTSC removing filter 200 removes an NTSC component from the IF signal to prevent degradation of an HDTV due to the NTSC component; an equalizer 300 removes multi-path distortion generated in a transmission channel by equalizing an input signal; a phase tracer 400 removes phase noise from the equalized signal; a trellis decoder 500 performs trellis decoding with respect to an output signal of the phase tracer 400, and outputs the decoded signal; a data de-interleaver 600 performs reverse interleaving to the input signal; an RS decoder 700 performs Reed-Solomon decoding to the input signal so as to generate error-corrected bit streams; and a de-randomizer 800 provides the decoded signal into other elements of the receiving system.
In FIG. 2, a detailed configuration of an ATSC VSB equalizer is illustrated. The equalizer 300 basically uses an LMS algorithm. In FIG. 2, a front filter 301 and a rear filter 302 remove an inter-symbol interference component, which generates distortion, from a received signal. In case of a blind mode, a quantization block 303 compares the signal outputted from the filters 301 and 302 with a predetermined threshold value and outputs hard-decision data. A training symbol storing block 304 stores a training data symbol which is perceived in a transmitting system. The training symbol is fetched into the rear filter 302 and an error signal calculating block 306 in a training mode, and it is used for calculating errors of the equalized signal. In case of a blind mode, the output signal of the quantization block 303 and the equalized signal are used for calculating errors. A switching block 305 passes the equalized signal or training symbol therethrough in accordance with the two modes. The calculated error is outputted to a tap coefficient updating block 307.
The tap coefficient updating block 307 receives an error signal and input signal of the filter 301, and updates the tap coefficient by using an LMS algorithm. Such an updating method is as shown in the following Equation 1.C(n+1)=C(n)+μx(n)e(n)  [Equation 1]
In Equation 1, c(n) is a coefficient of a filter, x(n) is a received signal, e(n) is an error signal calculated in the error signal calculating block 306, and μ is a step size.
As such a channel equalizer, a DFE (decision feedback equalizer) is widely used in a digital television receiver. When a decision error is not generated in a quantization block of the DFE, noise amplification at output of the equalizer, which becomes a problem in a conventional linear equalizer, does not appear when a rear filter removes ISI due to a symbol determined previously. For this reason, the DFE is widely used in a receiving system. Therefore, it is important that a decision error is not generated at output of a quantization block, so the DFE can be used effectively. For attaining this technical point, a method for reducing decision error by inserting a training symbol at a constant period has been used in a digital communication system, while willingly accepting a reduction in data transmission efficiency. However, not enough training symbols are inserted in most cases. The conventional DFE does not have a rapid enough convergent speed to compensate for distortion of a received signal that is generated due to inter-channel interference under a highly distorted communication environment such as indoor reception, mobile reception, or in a communication environment producing ghost images such as a broadcasting channel. Therefore, a better equalizer that has a rapid convergent speed even in a highly distorted communication environment and is efficient at dealing with distortion even in a data duration without the training symbol being inserted is required.